1. Field of the Invention
This invention relates to a linear amplifier which is used in a high frequency band and used mainly as a power amplifier.
2. Description of the Prior Art
A recent trend in communication system design has been to narrow the effective frequency band of a channel in order to attempt effective utilization of the frequency spectrum. When a frequency band of a channel is narrowed, deterioration of a transmission signal becomes a problem owing to the nonlinear characteristics of amplifiers typically used in such communication systems. The reason is that the nonlinear amplitude output characteristic of an amplifier (AM/AM) and drifting of an output signal phase of an amplifier relative to an input signal phase (AM/PM) generates intermodulation components of odd orders such as the third order, the fifth order and the like, and consequently interference with adjacent channels is easily generated.
Particularly in transmission systems utilizing narrow band QPSK (quaternary phase shift keying) modulation such as mobile telephone communications, it is necessary to use the nonlinear range of power amplifiers because of the required data transmission speeds, e.g. a wide dynamic range is required (on the order of 17 dB). Further, in terms of operating performance, the most efficient amplifier operation occurs in the saturation or nonlinear range of the input/output characteristics.
FIG. 1(a) is a circuit diagram showing a simplified constitution, for example, of a conventional amplifier using a field effect transistor (FET) shown in chapter 8 of the book "Microwave" (written by Eitaro Abe) published by Tokyo University Publishing Society. In FIG. 1(a), reference numeral 1 is an input terminal, 2 is an output terminal, 100 is a FET, 101 is an input impedance matching circuit, 102 is an output impedance matching circuit, 103 is a gate bias circuit, 104 is a gate voltage supply terminal, 105 is a drain bias circuit, and 106 is a drain voltage supply terminal. This FET amplifier exhibits input/output characteristics as shown in FIG. 2. Though the characteristic is linear within a small input region, it does not remain linear as the input increases, resulting in generation of output distortions. Though the output phase exhibits a small change with respect to the input phase a small input region similarly, it varies largely as the input power increases. In this way, distortions are generated in both the amplitude and phase of an input signal in a region where an input power is large. In a quaternary phase shift keying (QPSK) modulation method, a type of phase modulation method, since the amplitude of an input signal varies a modulation signal, the linearity of the amplifier is particularly important. FIG. 3 indicates the spectrum of an output wave signal obtained when a QPSK wave signal is inputted to an amplifier having a characteristic as shown in FIG. 2, from which it is seen that the quantity of leakage power interfering with adjacent channels (having adjacent frequency bands) is considerably large.
It is also possible to employ a bipolar transistor in place of the FET 100. A constitution of an amplifier using a bipolar transistor is shown in FIG. 1(b). In FIG. 1(b), reference numeral 107 is a bipolar transistor (an NPN type is illustrated in the figure) and reference numerals 101a to 106a are portions corresponding to portions 101 to 106 in FIG. 1(a). Specifically, reference numeral 101a is an input impedance matching circuit, 102a is an output impedance matching circuit, 103a is a base bias circuit, 104a is a base voltage supply terminal, 105a is a collector bias circuit, and 106a is a collector voltage supply terminal. The circuit shown in FIG. 1(b) operates in the same way as the circuit shown in FIG. 1(a), and its characteristic shows the same trend as that of the circuit shown in FIG. 1(a).
A conventional method for compensating a nonlinear characteristic of an amplifier is implemented by the high frequency amplifier circuit shown in Japanese Patent Disclosure Publication No. 274906/1987 or paper B-539 in the collection for the lecture at the national conference of The Institute of Electronics, Information and Communication Engineers (Japan) held in the fall of 1989. This amplifier circuit varies its drain voltage in proportion to an envelope level of an input signal and has the feature that even if the amplifier has a large distortion such as a class F amplifier, it exhibits an amplifier characteristic having good linearity and its power efficiency is thus enhanced irrespective of changes in the envelope level of its input signal.
FIG. 4 is a circuit diagram showing such a conventional high frequency amplifier circuit which so compensates a non-linear characteristic. In FIG. 4, reference numeral 1 is an input terminal, 2 is an output terminal, 3 is a DC voltage supply terminal, 10 is an amplifier, 20 is an envelope detector circuit, 30 is a non-linearity control circuit, and 40 is a voltage variable DC-DC converter. The above-mentioned envelope detector circuit 20, non-linearity control circuit 30 and voltage variable DC-DC converter 40 constitute an amplitude characteristic correction means. The non-linearity control circuit 30 is a circuit which outputs a drain voltage in accordance with an envelope level of an input signal based on look-up table data of an input-output amplitude characteristic at the time when a drain voltage of the amplifier 10 is varied. The look-up table is implemented by a ROM (Read Only Memory) or the like. In other words, the drain voltage values for which the input-output amplitude characteristics shown by the dashed lines in the upper graph of FIG. 5(a) become a straight line are stored in the ROM corresponding to the envelope levels of the input signal. The value of the drain voltage in accordance with the envelope level at that time is outputted by the control circuit 30. The voltage variable DC-DC converter 40 is a circuit which converts an output voltage from the non-linearity control circuit 30 into a corresponding drain voltage for operating the amplifier 10, and needs to operate at a high speed in order to follow changes in the envelope level of the input signal. In this circuit, even if the amplifier 10 is an amplifier having a large distortion such as a class F amplifier which characteristic is shown by the dashed lines in FIG. 5(a), the input-output amplitude characteristic can be made to be a substantially straight line by controlling the drain voltage in accordance with the envelope level of the input signal, and consequently the amplifier 10 can be operated linearly while retaining a high power efficiency. Also, the input-output phase characteristic is improved over the conventional amplifier, as shown by the solid line in the lower graph of FIG. 5(a). FIG. 5(b) shows the frequency spectrum of an output wave signal for input signal of a center frequency of 1.5 GHz. In FIG. 5(b), A is a spectrum based on the amplifier 10 without drain voltage control, and B is a spectrum in the case where the high frequency amplifier circuit shown in FIG. 4 is employed. It is seen that the spectrum B based on the high frequency amplifier shown in FIG. 4 exhibits lower levels of frequency distortion than the spectrum A and consequently, the leakage power leaked to adjacent channels becomes small.
However, in such a conventional high frequency amplifier as shown in FIG. 4, there is a problem that though the amplitude-amplitude (AM/AM) conversion of the amplifier can be corrected because the amplitude characteristic of the amplifier is made substantially linear, the amplitude-phase shift (AM/PM) conversion can not be corrected because the phase characteristic is not made flat (constant). For this reason, deterioration in the spectrum caused by phase distortions can not be prevented. Accordingly, a sufficient characteristic can not be obtained by this high frequency amplifier circuit in the case where leakage power leaked to adjacent channels is required to be very small.